Voltage drop above 4500 rpm

[truckmann]

Seems my alternator is shutting off on WOT runs at about 4500 rpms. Anyone seen this before or have any ideas why this might be happening? I went from a 105 amp alt to a 145 amp alternator. Thinking about trying a different alternator. Here is a short log showing the issue tho I doubt it will help to diagnose the problem. Are there any other controls in HPT regarding the alternator, either that I'm not seeing or can't see?

This is really annoying and I don't know what else to check.
All my grounds and wiring are good and have even been upgraded with larger wiring.

[truckmann]

Anyone?

[Bluecat]

I had a LT1 do that but it ended up being a bad alt. Also seen a 04 cobra do that, but it was from a lower pulley overspeeding the alt. And I've seen a bad 4l60 pull the voltage down and make the alt shoot sparks out of the case during a shift (don't laugh, it happened ) Never seen it on a LS.

The computer could be killing the field control to make more power above a certain rpm. Ford has had some clibrations like that I think. But I don't know of GM ever doing that, and I'm pretty sure they never did that with a LS calibration. Especiually a truck.

[truckmann]

I've actually heard about it on a couple other trucks with the 145 amp alt, but have not found any good answers on what the cause is. The alternator is only a few months old and I could warranty it out just to make sure it's not bad, but I don't think that the alt is the problem. I upgraded when my old 105 amp alt quit charging and I'm not sure when this actually started. I didn't notice it until putting on the TBSS intake and doing some logging in the dark and seeing the lights dim noticeably at WOT. I also had the battery load tested and was told that it is fine. I guess it's not hurting how it is running just kind of annoying seeing the lights all dim above 4500 rpm at night.

[jimmyblue]

I wonder, since PCM has the ability to meddle via
the {exciter?}, if there is some "strategy" to cut
accessory load by cutting the alt output. Kind of
like how the A/C compressor is shut down at RPM.
There seems to be a fair number of people seeing
this behavior, maybe the ones with so much acc
load that they sag on battery-only are the ones
who notice.

But I did have a bad alt on my Sportster that
would act this way, above certain RPM it would
go into brush-skip and quit charging.

[truckmann]

I'll double check the wiring this weekend.

I did find a wire with a damaged cover on the connector to the alternator after I first noticed the problem. The wire itself was fine so I sealed up the damage and hooked it back up.

[Mike@ADS]

You could also bypass the field exciter wire and set it up to a 12v source yourself temporarily to eliminate the ecu all together. I use an aftermarket ecu to tune all of the sandrails and this is always an issue. It is always with the truck 145 amp alternator as well. Most of them drop off at about 5k and are down to 10v or so at 7000 rpm at the ecu. They are already hooked to a switched ignition source at the exciter wire and still have this problem. It is a nightmare with the voltage drop. I don't see it with a new Corvette alt, only the truck and rebuilt units.

Mike

[truckmann]

Thanks for the info Mike that confirms my suspicion about the rebuilt truck alts. I made Autozone warranty out the alternator and the new one is better not perfect mind you but much better. It seems to hold good through most of a WOT run with an occasional voltage drop now. Of course who knows how long it will last before it starts acting up again. If that happens I will start looking for a better quality built new alternator.

[MikeGyver]

Wow, I just found this thread. I have the same problem and just started searching. The guy that sold me the alternator suggested belt slip. That sounded likely until I started thinking about it, that's when I came here to ask the same question. I wouldn't worry about it except the fuel pump slows down, the fuel pressure drops, and the AFR starts leaning out. At the most critical time.
So is the only answer a new Corvette alternator?

[truckmann]

I would agree with that. I took my alt back and made them warranty it even though there lousy machine couldn't test it. I had to take it over to O'Rielly's and have it tested where it passed the test on the piddly ass machine at I'm sure less than 4500 rpm. The guy there told me they would warranty it anyway without question if I wasn't happy. The chick at autozone bitched and moaned that I wanted a new one without some kind of proof it was faulty, but she finally gave me a new one anyway. The new one is much better so far.

The only reason I bought it from Autozone to begin with was that no one else had a 145 amp in stock when my old one crapped out and I need it fixed asap. I'm not a fan of Autozone.

[MikeGyver]

I got a new Bosch 130A. Everything is fine now. Any rpm, 14.1-14.3 volts. The fuel pressure graph is flat at WOT, as is the AFR. I've always had an issue that the engine dies when the electric fans start, while the engine is warming up. The fan controller starts the fans, both on high speed, about 30 seconds after starting the engine. It is just something it does. Anyway, now the engine isn't even affected.

[MikeGyver]

Anybody want to buy a used alternator?

[Dragman]

no, don't buy autozone alt.. lol go with oem or oem quality, napa auto parts..

truth!

[Mike@ADS]

Heck, I've even had problems with the rebuilt Delco ones. I normally have to go through 2 or 3 of those before I find a good one. Only buy new Delco ones. Unfortunately most others are crap. The bad thing is the fact that the shops that test them can't spin them fast enough and always say they are bad. It is normally inferior brushes that start arcing out and break up at high rpm's only that cause the problem. Occassionally it goes away after full break in, but typically not. I'm glad you got it straightened out.
Mike